1. Field of the Invention
The present invention relates to an electroplating cell, and a metal coating and a method of forming the same, and more specifically relates to an electroplating cell which is capable of easily forming a metal coating on a surface of a cathode (plated object), a metal coating which is formed using the electroplating cell, and a method of forming the metal coating.
2. Description of Related Art
A technique of forming a pattern formed of a metal coating (hereinafter, referred to as “metal pattern”) on a conductive substrate with a simple method is required. A technique of masking a portion other than a metal pattern to perform wet electroplating is most commonly used. However, in this technique, a mask forming step and a mask removing step are required, and there is a problem in that the cost for the management and waste liquid treatment of a plating solution is high. Recently, a method of forming a metal coating with a “physical method” such as physical vapor deposition or sputtering not having the above-described problem and then removing a masking portion has been adopted. However, in this method of physically forming a metal coating, a film forming speed is generally slow, and a vacuum unit is necessary. Therefore, it is difficult to say that a system using this method is an economical high-speed production system.
On the other hand, as another method in which masking is not necessary, a method of coating a substrate with an ink, in which conductive fine powder and a binder are mixed, using “printing method” such as screen printing or an ink jet and then removing the binder has been also adopted. However, with this “printing method”, it is difficult to form a circuit having low volume resistivity even if a volatile or sublimable binder is used.
However, recently, as an attempt to form a circuit without masking in electroplating, a gel electrolyte (Japanese Patent Application Publication No. 2005-248319 (JP 2005-248319 A)) and a technique using a separator such as a solid electrolyte membrane (Japanese Patent Application Publication No. 2012-219362 (JP 2012-219362 A) and International Publication WO 2013/12563) have been proposed.
When such a separator is used, a current density of approximately 10 mA/cm2 is obtained at room temperature in Cu plating in which electrodeposition from an aqueous solution is relatively easy. However, in a film forming process (high current density electrodeposition) in which a higher speed than that of the Cu plating is required, it is necessary to take an action, for example, to increase a metal ion concentration or to increase the temperature. Therefore, a higher cost is required. In particular, it is difficult to electrodeposit metal (for example, metal in which deposition potential of nickel ions, zinc ions, tin ions, or the like is low), in which an electrodeposition reaction (reduction deposition reaction) competes with a H+ ion discharge reaction (hydrogen evolution reaction), from an acidic or slightly acidic aqueous solution having high H+ concentration using a separator.
The details of the reason for this phenomenon is unclear, but it is considered that this phenomenon is caused by the following reasons (1) to (3).
(1) Hydrogen is produced at an electrodeposition portion, and defects (voids) are formed.
(2) Due to deposition over voltage being too low, metal is electroplated in a fine powder form or in a lump, and when the electrodeposition is performed in a state where a separator and a cathode are in close contact with each other, an electrodeposit infiltrates into the separator.
(3) Due to a pH increase caused by hydrogen production, a hydroxide is produced, and passivation (increase in bath voltage) progresses.
In order to solve the above-described problems, as in ordinary electroplating, a technique of adding, for example, an “organic plating additive” for improving physical properties of a deposited coating or suppressing hydrogen evolution reaction to a plating bath in which a separator is used is considered to be adopted. However, when the organic additive is added to the plating bath, it is necessary to accurately control the concentration of the organic additive. Further, since the organic additive is decomposed and consumed on an electrode, it is also necessary to remove a waste product. In addition, when a large amount of the organic additive is added to the plating bath, there is a problem in that electrodeposition efficiency is likely to decrease.
Furthermore, when the separator is an ion exchange membrane, and when the organic additive is an ionic compound, there is a problem in that the organic additive is adsorbed onto the separator, conductivity decreases, and bath voltage increases. For example, when metal ions in the plating bath are cations (positive ions), it is advantageous to use a solid electrolyte membrane (cation exchange membrane) having a high cation transport number as the separator from the viewpoint of high-speed plating. Nevertheless, in electroplating using a cation exchange membrane, it is common to adopt semi-bright plating not using an additive (Japanese Patent Application Publication No. 2009-173992 (JP 2009-173992 A)) or to use a non-ionic (neutral) separator having low risk of membrane fouling by the organic additive.
As the organic additive, it is recommended to use a non-ionic surfactant which is weak in coating physical property improving effect but is not likely to be adsorbed onto a cation exchange membrane (Japanese Patent Application Publication No. 2007-002274 (JP 2007-002274 A)) or to use a neutral additive (Published Japanese Translation of PCT application No. 2007-523996 (JP-A-2007-523996). Alternatively, it is recommended to laminate a neutral separator on a cathode chamber side of a cation exchange membrane and to add an organic additive to only a cathode chamber solution such that the organic additive is not adsorbed onto the cation exchange membrane (Japanese Patent Application Publication No. 2008-038208 (JP 2008-038208 A)). In the above-described techniques, an attempt to actively hold an organic plating additive inside a separator (in particular, a solid electrolyte membrane such as a cation exchange membrane) is not made.